A push-pull chain can be extended and retracted by an actuator basically comprising a driving sprocket and a housing for the push-pull chain. The driving sprocket engages the push-pull chain's rollers and drives it in the desired direction. In order to keep these actuators compact the sprocket is provided with as few teeth as possible. However, typically the sprocket has five teeth since the so called polygon effect becomes too large and disruptive for the actuator operation with fewer than five teeth.
It is a well known problem that the polygon effect even with five or more teeth causes an irregular chain speed (assuming that the sprocket rotates with a substantially constant speed). The irregular chain speed results in irregular forces on the chain and the actuator and on the object to be actuated and causes noise, wear and reduced performance. Various attempts have been made in the prior art to overcome this problem. One of such attempts is disclosed in EP 1 227 056. In this document it is proposed to use noncircular gear wheels that are configured to offset the polygon effect. Thus, the sprocket will not rotate with the constant speed, but rather with the fluctuating speed, whereby the fluctuations are configured and dimensioned to ensure that the resulting chain speed is constant. However, noncircular gear wheels are more complicated to produce and cannot be obtained off the shelf from mass production. Thus, this solution is relatively expensive.
Another issue with roller chain actuators is there need to be provided with a chain exit guide that least supports two rollers at the time, otherwise the chain becomes instable. The links of the chain exit guide is a limiting factor in reducing the size of such actuators.
One way of reducing the noise has been discussed in U.S. Pat. No. 4,827,668 that disclose a chain operator having a chain and a casing having a spaced first and a second exit opening. The chain is guided by a track from the first exit to the second. The casing also has a drive sprocket for driving the chain. This guide is used to steer the chain correctly so that the sprocket teeth do not jam into the chain links this to avoid unwanted noise. This apparatus is difficult to install as it is large, the push-pull chain is prone to rattle and the engine sound is uneven. As the casing is used to muffle the engine sound it needs to substantially completely enclose the actuator components and needs to be of a thick material to effectively silence the actuator.
As the push-pull chain is moved by the sprocket it goes from a linear motion, through a circular motion and back to a linear motion again. It is in this point of circular motion that the varying contact arises partly due to the stiffness of the individual links.
Another problem is that as the push-pull chain is under push load, as e.g. in a push-pull chain being used to open a window, the push-pull chain is urged away from the sprocket due to the push force exerted on the push-pull chain by the load. This effect may cause the push-pull chain to disengage the sprocket completely or partly leading to noise and partially or completely impaired function.
To overcome this, a first guide surface is used to guide the push-pull chain along the sprocket and prevents the push-pull chain from moving away from the sprocket and keeps the push-pull chain in close relation to the sprocket.
Actuators having guide surfaces such as these, however, still make a lot of noise and the push-pull chain is prone to rattle, in particular due to a swaying movement of the push-pull chain in the vicinity of the chain exit caused by the interaction of the sprocket with the push-pull chain.
In a small actuator a small sprocket is required and this leads to a limited number of teeth for a robust push-pull chain. Due to the size and geometry of the teeth only 1 to 2 chain links are in contact with the sprocket. For a sprocket with 5 teeth usually 1-1% chain links are in contact with the sprocket at any given moment in time.
As the push-pull chain progresses along the sprocket it will first come in contact with the sprocket, follow it and then leave it. Doing so the rollers 8b of the push-pull chain are being engaged by different parts of the sprocket, i.e. the teeth of and the recesses in-between them. As the recesses are closer to the centre of the sprocket than the teeth, the sprocket has a polygonal geometry, and when the sprocket is rotating with a steady speed the resulting chain velocity is varying. Also the force transmitted from the sprocket to the push-pull chain will be un-even in both direction and amplitude. This is especially apparent when the push-pull chain is put under load and thereby urged away from the sprocket. Due to this polygon effect the push-pull chain will leave the actuator at the chain exit opening with varying angle, and the push-pull chain will sway or oscillate. For a sprocket only having a small number of teeth this problem becomes more apparent as the polygonal effect becomes more prominent as only a few links are in contact with the sprocket at any time which causes the effect of a single tooth to dominate the movement of the link and since the changes in the effective radius of the sprocket become more prominent. This causes the push-pull chain to move erratically which causes it to sway as it leaves the actuator. This swaying movement reduces the push-pull chain stability under push load as it increases the risk of the push-pull chain assuming a shape in which it is prone to collapse. Furthermore, rattling noise is generated as the push-pull chain clangs against the actuator and other parts and also by the push-pull chain itself. Especially for push-pull chains the stability is an important issue.
Another problem associated with chain actuators are the chain oscillations that are caused by the rollers leaving or entering the chain exit/entry opening. A transverse force is unavoidably applied to the chain and each time a roller enters or leaves the chain exit/entry there is a transverse movement of the roller that is getting into or out of contact with the chain guide. Theoretically, this problem could be reduced if the actuator and the chain could be constructed without any play or backlash between the rollers and the chain guide and between the rollers and the pins of the chain. However, in practise a certain amount of play is required and has to be accepted. The resulting chain oscillations result in noise and war and reduced performance and are therefore undesirable.